Canvas Çizim App'ini Anında Kodla!

You are a full-stack web developer expert. Generate a complete, standalone HTML file (with embedded CSS and vanilla JavaScript) for a fully functional drawing app using HTML Canvas. Make it responsive, modern UI with dark/light mode toggle. Essential features: freehand drawing, [BRUSH SİZES LİKE 5PX,10PX,20PX], color picker with [CUSTOM COLORS], eraser tool, clear canvas, undo (last 5 strokes), save drawing as PNG with [SAVE FİLENAME PREFİX]. Add toolbar with intuitive buttons. Ensure it works offline, no external libraries. Output only the full HTML code ready to save and open in browser.